Method and intermediate product for manufacturing multicore fibers having markers

Abstract

This invention provides a method for manufacturing a multicore fiber having a marker zone. [Solution] A method for manufacturing a multicore fiber or a preform for a multicore fiber, comprising forming a semi-finished product comprising a glass-clad region in which a plurality of core glass regions are embedded and at least one marker element, wherein the multicore fiber or preform is obtained by stretching the semi-finished product, and the manufacturing of the glass-clad region comprises a method step of depositing clad material onto a target rod by an external deposition method, wherein the target rod comprises a first glass region extending along a longitudinal axis and a marker region extending along a longitudinal axis and adjacent to the first glass region, wherein the marker region comprises or forms a marker element or provides a hollow channel, and the semi-finished product comprises a clad material layer and a target rod.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a multi-core fiber having a marker zone or a perform for such a multi-core fiber, comprising forming a semi-finished product including a glass cladding region made of cladding glass in which a plurality of core glass regions made of core glass are embedded and at least one marker element, wherein the multi-core fiber or perform is obtained by stretching the semi-finished product, and the manufacturing of the glass cladding region includes a method step of depositing a cladding material layer on the outer surface of a target rod having a longitudinal axis of the target rod using an external deposition method.

[0002] Furthermore, the present invention relates to an intermediate product for manufacturing a multi-core fiber or a perform thereof.

[0003] In a multi-core fiber, a plurality of optical wave-guiding light core regions (hereinafter also referred to as "signal cores") are integrated into a common fiber. The signal cores extend along the longitudinal axis of the fiber. They are surrounded by a cladding material having a lower refractive index, enabling substantially independent light guidance. This fiber design ensures a high signal transmission capacity since different signals combined within a single optical fiber can be simultaneously transmitted in each of the spatially separated signal cores. This signal transmission method is also referred to as "space multiplexing" and can particularly increase the data transmission capacity in optical communication. Multi-core fibers are also regarded as components of optical fiber sensors in measurement and medical technology and are being considered for the purposes of illumination and imaging in microscopes or endoscope devices.

Background Art

[0004] Multicore fibers are manufactured by stretching a solid preform or group of components. These often consist of synthetically produced quartz glass (SiO2), which may be doped or undoped. The production of synthetic quartz glass includes plasma or CVD deposition methods, known, for example, as OVD, VAD, MCVD, PCVD, or FCVD methods. A liquid or gaseous silicon-containing starting material is subjected to a chemical reaction (hydrolysis, thermal decomposition, or oxidation) to deposit particulate SiO2, the reaction product, from the gas phase to a solid on the deposition surface. The starting material is, for example, silicon tetrachloride (SiCl4) or a chlorine-free silicon compound such as a polyalkylsiloxane. The reaction zone is, for example, a furnace, a burner flame, or an arc (plasma).

[0005] In the so-called "stack-and-draw" method, core rods and glass cylinders of different diameters are stacked, thereby creating a relatively high packing density and a certain degree of symmetry. The cylindrical components are inserted into a sheath tube and spatially fixed within it. This group is then drawn to form multicore fibers, or further processed to form preforms from which multicore fibers are drawn. This method is described, for example, in U.S. Patent No. 6,154,594(A).

[0006] The "stack-and-draw" method requires a high level of adjustment effort, is prone to errors in dimensional stability, and is susceptible to impurity inclusion due to the large proportion of free-standing component surfaces. Furthermore, differences in radial packing density often result in stretched preforms having different radial values ​​in the azimuthal direction, which must be compensated for by cylindrical grinding.

[0007] In the method known from U.S. Patent Application Publication 2015 / 0307387(A1), preforms for multicore fibers are manufactured by drilling through holes in a glass rod-shaped substrate and then inserting core glass rods into the through holes. A hollow glass-clad cylinder in the form of an SiO2-based soot body manufactured according to the OVD method is used as the substrate (SiO2 soot body). In the OVD method, the deposition surface is generally the outer surface of a rod-shaped or tubular deposition mandrel that rotates around its longitudinal axis. The substantially cylindrical soot body reverses the forward and backward motion of the reaction zone. Thus, it is deposited. After the deposition process is complete, the deposition mandrel is removed, resulting in a central intermediate bore remaining on the central axis of the cylindrical suit body. A longitudinal bore for receiving the core glass rod is introduced into the hollow glass-clad cylinder. Furthermore, an additional longitudinal bore for receiving marker elements in the form of marker rods is formed by mechanical drilling in the region of the hollow glass-clad cylinder near the edge.

[0008] When multicore fibers are drawn, marker rods are stretched to form marker zones. Marker zones are particularly useful in breaking symmetry to identify the signal cores of multicore fibers and to clearly assign them with respect to their relative positions and fiber central axes, especially in symmetric fiber designs. Identifying and assigning signal cores is necessary, for example, to allow two multicore fibers to be joined together via end faces by conventional splicing methods that are low-attenuating and correctly assign the signal cores. [Overview of the project] [Problems that the invention aims to solve]

[0009] The production of hollow glass-clad cylinders using external deposition (OVD) methods is particularly cost-effective compared to other manufacturing methods, especially VAD (Vapor-Axis Deposition). However, it has the drawback of leaving a central bore after removal of the deposition mandrel, which leads to mechanical stress and asymmetric deformation during collapse and can also destroy the fiber design. Furthermore, this collapse reduces the cross-sectional area of ​​the glass-clad portion. These drawbacks can be at least partially mitigated by inserting a filling rod to close the central bore. The filling rod can be made of glass having substantially the same refractive index and coefficient of thermal expansion as the hollow glass-clad cylinder. However, even with this measure, mechanical stress may be generated in the filling rod during collapse, potentially requiring subsequent tempering of the composite formed by the hollow glass-clad cylinder and the filling rod.

[0010] Marker zones, which are additionally inserted into the fiber design, need to have the smallest possible cross-section to counteract undesirable effects such as impact on signal transmission, stress induced within the fiber, or so-called fiber curl. The degree of curvature over a certain length of the fiber is defined as "fiber curl." This curvature is caused by thermal stress that occurs during fiber manufacturing. Large fiber curls result in optical loss due to microbends, making low-attenuation splicing of multicore fibers more difficult.

[0011] To minimize these drawbacks, a thin marker zone is required, which necessitates making the diameter of the channel for receiving the marker element within the plane of the hollow glass-clad cylinder as small as possible. Typically, the channel diameter in a hollow glass-clad cylinder is less than 15 mm, with a high aspect ratio of over 65 (the length of the hollow cylinder is approximately 1 m).

[0012] Manufacturing and precisely aligning such thin channels within hollow glass-clad cylinders is difficult, even when using high-precision drilling machines. Furthermore, it has been shown that cracks increasingly occur in the channel walls, especially during the drilling of thin channels. The cost of manufacturing hollow glass-clad cylinders from synthetic quartz glass is high, and above all, the small bore required to accept the marker element means that if an otherwise completed hollow glass-clad cylinder is rejected, the loss is particularly painful.

[0013] Furthermore, since the cutting and splicing of multicore fibers can be performed at any desired location according to the specific requirements of the intended use, it is necessary to be familiar with a consistent shape along the entire length of the fiber, in a manner independent of the specific procedure. This means that the longitudinal axes of the marker elements and the hollow glass-clad cylinder must extend as parallel as possible within the plane of the hollow glass-clad cylinder.

[0014] Therefore, an object of the present invention is to identify a method for manufacturing multicore fibers having marker zones, which takes advantage of the cost benefits of external deposition methods while reducing the drawbacks and difficulties associated with packing rods and marker elements, and also reduces the risk of rejection.

[0015] Furthermore, an object of the present invention is to provide an intermediate product suitable for cost-effectively manufacturing multicore fibers having marker elements, particularly characterized by low fiber curl. [Modes for carrying out the invention]

[0016] With respect to this method, this objective is achieved according to the present invention beginning with the method described at the beginning, wherein the target rod has a first glass region extending along the longitudinal axis of the target rod and a marker region extending along the longitudinal axis of the target rod and adjacent to the first glass region, the marker region providing a hollow channel that includes, forms, or receives a marker element, and the semi-finished product includes a clad material layer and the target rod.

[0017] Multicore fibers are obtained by stretching preforms or semi-finished products. Semi-finished products are, for example, primary preforms for multicore fibers, or groups of components that are directly drawn to form multicore fibers, or further processed to form preforms for multicore fibers.

[0018] The production of a semi-finished glass-clad region includes a method step of producing a clad material layer using an external deposition method. External deposition methods include, for example, thermal spraying or vapor deposition.

[0019] In thermal spraying, an oxidizable or slightly oxidizable silicon-containing or silicon dioxide-containing starting powder in the form of a fluid mass such as a flowable SiO2 powder, sol, dispersion, or slurry is supplied to an energy source, melted in it, and spun at high speed onto the outer surface of a deposition mandrel rotating around its longitudinal axis. The energy source is, for example, a flammable gas-oxygen flame, a plasma jet, an arc, or a laser beam. In this case, plasma spraying is particularly preferred as it allows for relatively high energy input and high speed during the spin coating of the molten starting powder particles.

[0020] In vapor deposition (VPD), SiO2 particles are produced by in-situ hydrolysis, thermal decomposition, or oxidation of silicon-containing precursors, which are then deposited as an SiO2-containing layer on the outer surface of a deposition mandrel rotating around its longitudinal axis. Examples include OVD (outer vapor deposition) and POD (plasma outer deposition). At sufficiently high temperatures in the region of the deposition mandrel surface, the SiO2 particles are directly vitrified, also known as "direct vitrification." In contrast, in a method known as the "soot method," the temperature during deposition of SiO2 particles is lower, resulting in a porous SiO2 soot layer.

[0021] The porous SiO2 soot layer obtained as a cladding material layer by the soot process is vitrified in a separate method step to form a glass-clad region made of transparent quartz glass. In the case of external deposition methods using direct glazing and thermal spraying, the glass layer is obtained directly as a cladding material layer, forming a semi-finished glass-clad region or at least a part thereof.

[0022] The production of semi-finished glass-clad regions using external deposition methods is more cost-effective compared to other manufacturing methods.

[0023] In the method according to the present invention, a target rod is used as the deposition mandrel for the external deposition method. After the completion of the external deposition method, the target rod remains in the deposited clad material layer, thereby forming part of the semi-finished product for manufacturing multicore fibers. The target rod is made of a glassy material that is part of the multicore fiber. This differs from other external deposition methods that leave a central through-opening in the clad material layer by removing the deposition mandrel after the completion of the external deposition method. Therefore, subsequent filling of the through-opening with a filling rod is unnecessary, thus eliminating the difficulties associated with filling in order to ensure straightness, dimensional stability, and maximum relief from stress, as well as reducing the risk of failure and the costs of adjustment, time, and materials.

[0024] The target rod has at least one first glass region and an adjacent marker region. The marker region is formed within or on the target rod. Thus, the target rod is used to insert marker elements into the semi-finished product in addition to the target rod material. In addition to the first glass region, the target rod can have further glass regions or plural glass regions that are different from the first glass region in terms of chemical composition.

[0025] In the target rod, the marker element can form, for example, a hollow channel filled with air, a cylindrical component or plural cylindrical components or a powder bed, or the marker element is arranged on the outer wall of the target rod, where it is designed as (at least one) cylindrical component or as a coating or mass adhering to the outer wall. After completion of the external deposition process, the hollow channel can also be filled with the marker material.

[0026] Since the marker element is arranged within or on the target rod, it is not necessary to adapt the cladding material layer for the purpose of inserting the marker element, for example by machining, particularly by forming a bore for receiving the marker element in the cladding material layer. Thus, the labor and the risk of damage associated with such adaptation of the cladding material layer are eliminated.

[0027] The dimensional stability and straightness of the target rod can be relatively easily ensured as required by mechanical grinding and / or a stretching process (subjecting the starting cylinder to a stretching process to stretch a cylinder strand therefrom and forming the target rod from the cylinder strand or cutting plural target rods from the cylinder strand to a predetermined length). To avoid damage to the surface of the cylinder strand, the stretching process is preferably carried out without using a shaping tool acting on the drawn cylinder strand.

[0028] Alignment parallel to the axial direction of the marker element is facilitated by the marker element being adjacent to the target rod or its first glass region. Its straightness is relatively easy to achieve and also facilitates the axial alignment of the marker element. This applies particularly to a particularly preferred procedure of arranging the marker region between the first glass region and the glass cladding region.

[0029] Thus, the component group comprises a marker element without the need to form separate bores in the glass cladding region associated with the risks and difficulties described above. At the same time, high accuracy can be ensured despite the high aspect ratio, which becomes apparent, for example in a semi-finished product, from the deviation of the axial parallelism of the marker element being less than 0.3 mm / m.

[0030] The target rod can consist of a single glass cylinder or can be composed of a plurality of glass cylinders connected to each other. The plurality of glass cylinders may have the same composition or may have different compositions from each other. The target rod can consist entirely or partially of cladding glass and / or core glass, or of another glass and / or other glasses.

[0031] The chemical compositions of the first glass region and the adjacent marker region are different from each other. In a multi-core fiber, the marker region forms a continuous linear marker zone made of a marker material or air. The marker zone can serve, for example, to break symmetry during splicing and also to clearly identify the signal cores and their positions relative to each other and relative to the fiber central axis.

[0032] In the cladding material layer, a plurality of core rod bores for receiving each core rod are formed in a conventional manner. It is also possible to introduce at least one core rod bore for receiving a core rod into the target rod.

[0033] The semi-finished product manufactured in this manner, having multiple core glass regions, is reshaped and directly drawn to form multicore fibers, or integrated to form a preform for multicore fibers, the integration process of which may involve simultaneous stretching. The "integrated preform" manufactured in this manner is optionally drawn to form multicore fibers, or further processed to form a "secondary preform". Further processing to form a "secondary preform" includes, for example, forming additional bores in the glass clad region and coating the bores with core glass or other glass, or performing one or more of the hot forming processes of collapse, collapse, stretching, collapse and simultaneous stretching of additional clad material, once or repeatedly. Multicore fibers are drawn from the secondary preform manufactured by the further processing.

[0034] The marker element is preferably provided in the form of a cylindrical component made of marker material, or in the form of a layer connected to a target rod, or a mass made of marker material.

[0035] In the case of a marker element including at least one cylindrical component, this component extends parallel to the target rod and is connected to the target rod at least locally, preferably along the entire length of the component. The at least one cylindrical marker element component is, for example, a tube, preferably a rod. In the case of a marker element in the form of a tube, the tube wall may contain a material with a higher viscosity than clad glass, so that the bore does not completely collapse during the fiber drawing process and is maintained as a cavity ("airline") within the finished multicore fiber.

[0036] In the case of marker elements in the form of layers or blocks connected to a target rod, the layers or blocks are, for example, located within a hollow channel of the target rod and preferably attached to an area of ​​the outer surface of the target rod.

[0037] By attaching the target rod, the marker element benefits particularly from its straightness and alignment, and these properties are transferred to the marker element to some extent. This attachment is based, for example, on frictional engagement, integral coupling, and / or shape fitting between the target rod and the marker element.

[0038] The cross-sectional shape of the target rod is generally circular. However, it may also have shapes that deviate from a circle, such as oval, elliptical, or polygonal. The surrounding circle enclosing the cross-sectional contour has a diameter in the range of, for example, 36 mm to 76 mm.

[0039] The target rod is manufactured, for example, by depositing it axially from the gas phase according to the so-called VAD or OVD method, and then collapsing the central through-opening, or by a pressing method. In the pressing method, a bed of SiO2 particles is introduced into a mold cavity, pressure is applied to the bed to form a compressed blank, and then the compressed blank is vitrified to form a glass rod.

[0040] In a preferred procedure, the target rod has a recess extending along the longitudinal axis of the target rod, the recess forming a marker element, or a marker element being positioned within the recess.

[0041] The recess is designed, for example, as a bore that penetrates the target rod, preferably as a longitudinal groove (longitudinal channel) on the outer surface of the target rod. The groove-like recess is filled, for example, with a cylindrical component made of marker material or particulate marker material. The particulate marker material can have a certain degree of dimensional stability by thermal compression or by the addition of a binder. This ensures that the recess is shape-fitted between the marker element and the target rod. The marker element and the target rod can also be connected to each other in advance (i.e., before the deposition of the cladding material layer) by, for example, sintering or fusion, which is referred to herein as "integration". Any edges and protrusions can be rounded during integration. After integration, it is preferable that the marker material fills the recess as completely as possible.

[0042] In this regard, the procedure of providing longitudinal grooves on the target rod is particularly preferred. This is because, on the one hand, longitudinal grooves are particularly easy to manufacture geometrically accurately on the outer surface of the target rod compared to the bore, for example by milling with a mechanical milling machine or by laser ablation. On the other hand, the longitudinal grooves thus manufactured are just as accurate and straight as the target rod itself. Furthermore, the depth or opening width of the longitudinal grooves can be substantially as small as desired, for example, both less than 15 mm, preferably less than 10 mm, and particularly preferably less than 5 mm. During the external deposition method, the longitudinal grooves can be filled with marker material. Thus, geometrically accurate marker elements can be manufactured in a small volume in a simple manner, and the marker elements have an axial deviation of less than 0.3 mm / m in the semi-finished product, and therefore form small, high-precision marker zones within the multicore fiber accordingly. The longitudinal grooves are filled with marker material, for example, by inserting cylindrical components (rods or tubes) made of the marker material, or by introducing a bed of marker material particles, or by coating the inside of the longitudinal grooves with the marker material. Floors formed from cylindrical components, internal coatings, or marker materials can be further fixed to longitudinal grooves by fusion.

[0043] In a preferred procedure, the fused composite of the target rod and marker element is manufactured by stretching a pre-product. The pre-product exists, for example, as a composite of the target rod and marker element, or forms a group of target rod and marker element starting materials that are not connected to each other or are only locally connected to each other. The pre-product has a larger lateral dimension than the manufactured fused composite consisting of the target rod and marker element. Stretching the pre-product reduces any geometric errors and dimensional deviations, on the one hand, and on the other hand, this stretching contributes to the straightening of the fused composite. The desired fused composite formed from the target rod and marker element is cut to a predetermined length from the stretched fused composite strand for use in the manufacture of multicore fibers. In this way, marker elements with small lateral dimensions can be manufactured with high dimensional stability, especially even with large aspect ratios.

[0044] In a particularly preferred modification of the method, the cladding material layer, manufactured using an external deposition method, is formed as an SiO2-based soot layer.

[0045] The SiO2 soot layer is obtained by hydrolysis, thermal decomposition, or oxidation of a silicon-containing starting compound in an oxygen-containing reaction zone, and by depositing the SiO2 particles formed during this process onto the outer surface of a rotating target rod. Here, the temperature of the region on the outer surface of the target rod is kept low, so the deposited cladding material layer does not remain densely vitrified, but rather becomes porous. This temperature is, for example, in the range of 800°C to 1250°C, and the specific density is typically about 0.6 g / cm³. 3 ~1.8g / cm 3 It is within the range.

[0046] Due to the porosity of the SiO2 clad material layer, the OH groups contained therein can be removed by subsequent treatment in a dry atmosphere and doping with a gaseous dopant, such as fluorine.

[0047] The soot layer is preferably vitrified by heating it to the vitrification temperature.

[0048] Vitrification is carried out, for example, by heating to a temperature exceeding 1400°C in a sintering furnace under vacuum and / or in a helium atmosphere. In this way, a cladding material layer formed from transparent quartz glass is obtained.

[0049] In one procedure, prior to vitrification, the soot layer undergoes doping in a dopant-containing atmosphere and / or dehydration in a halogen-containing atmosphere or under vacuum.

[0050] Doping or dehydration can be performed immediately before the vitrification process and in the same furnace. Dehydration may include, for example, helium purging followed by a high-temperature chlorination process at a temperature of approximately 900°C. In an alternative drying under vacuum, the SiO2 soot layer is treated in a vacuum furnace at a pressure of 0.1 mbar or less and a temperature of at least 1150°C. This reduces the hydroxyl group concentration to less than 1 ppm by weight.

[0051] The doping process includes, for example, supporting fluorine on an SiO2 soot layer, in which the target rod is introduced into a doping furnace together with the cladding material layer and exposed to an atmosphere at a temperature exceeding 980°C, which contains a fluorine-containing substance such as silicon tetrachloride. By supporting fluorine on the SiO2 soot layer in this way, an average fluorine content of at least 1500 ppm by weight after integration becomes possible.

[0052] In a modified version of the preferred method, a core rod bore for receiving a core glass rod within the clad material layer is formed after the soot layer has been vitrified.

[0053] Compared to high-density quartz glass, the lower density of the cladding material layer formed from SiO2 soot facilitates the formation of bores for receiving core glass rods. Two or more longitudinal bores (core rod bores) are conventionally created in the cladding material layer, for example, 4 to 7, with their longitudinal axes extending parallel to the longitudinal axis of the target rod. The core rod bores are through bores or dead bores, and in each case serve to receive at least one core rod made of core glass.

[0054] The composition of the core glass is uniformly homogeneous when viewed radially, or it changes gradually or in steps. This differs from that of clad glass in order to ensure light induction in the core glass region. In the simplest case, all core rods have the same dimensions and are made of the same core glass. However, the core rods may also differ in their dimensions and / or the composition of the corresponding core glass.

[0055] To minimize the risk of damage to the core rod, the core rod is preferably inserted into the core rod bore after the porous cladding material layer has been vitrified and the glass cladding region has been formed from clear quartz glass.

[0056] The marker element forms an elongated hollow channel filled with air, or preferably comprises a marker material having at least one physical and / or chemical property different from that of the adjacent clad glass and / or adjacent first glass region of the target rod, the property preferably selected from refractive index, color, fluorescence, and / or glass specific density.

[0057] The properties (or multiple properties) that distinguish the marker element from the glass of the constituent group particularly affect the visual appearance of the marker element and are preferably detectable by a light sensor. For example, the glass composition of the marker glass, like the glass filler material, can be based on quartz glass. The refractive index of quartz glass can be altered by doping. For example, doping marker quartz glass with fluorine lowers the refractive index compared to undoped quartz glass. If carbon is mixed into the marker quartz glass, it may turn black. Depending on the oxidation state, doping marker quartz glass with titanium results in a grayish-blue color. Doping marker quartz glass with rare earth metals or germanium oxide causes fluorescence at wavelengths specific to the dopant. The glass specific density of the marker element can be altered by pore size, which manifests as reduced light transmittance compared to bubble-free glass.

[0058] In a preferred procedure, the manufacturing of the component group is carried out in the following method steps: (a) Providing a target rod including a marker region, (b) The step of providing a plurality of core rods including core glass, (c) A step of depositing a cladding material layer on the outer surface of the target rod using an external deposition method, (d) The step of forming a core rod bore in at least a cladding material layer and optionally in a first glass region of the target rod, (e) the step of inserting the core rod into the core rod bore.

[0059] The semi-finished product thus manufactured includes a target rod (having at least one marker element and a cladding material layer formed thereon) and a core rod. List numbers (a) to (d) do not indicate the order of the method steps. The semi-finished core rod is also referred to here as the core rod if it has already been fused within the corresponding core rod bore.

[0060] In a modified version of the preferred method, the marker region is formed as a hollow channel, and a cylindrical marker element is inserted into the hollow channel before or after the core rod is inserted into the core rod bore.

[0061] With respect to intermediate products for manufacturing multicore fibers or preforms thereof, the above technical problems are solved by the present invention, the intermediate product comprising a glass target rod, a marker element formed on or within the target rod, and a cladding material layer containing SiO2 soot, wherein the cladding material surrounds the target rod and the marker element and is connected to the target rod by friction engagement, shape fitting and / or integral bonding.

[0062] The intermediate product according to the present invention may thus appear in connection with the manufacture of multicore fibers or preforms for multicore fibers based on the method according to the present invention. In this application, this is a particularly preferred intermediate product.

[0063] The intermediate product is a cylindrical bonded composite comprising a clad material layer covered by a glass target rod and having a central region formed on or within at least one marker element. The clad material layer exists as a porous soot layer based on SiO2. It is manufactured by external deposition (OVD) and forms a glass-clad region made of clad glass on the final multicore fiber.

[0064] The target rod extends along the longitudinal axis of the cylinder and forms the central volume region of the cylinder. The central volume region also includes at least one marker element adjacent to the first glass region of the target rod.

[0065] In embodiments where the chemical composition of the first glass region of the target rod corresponds to the chemical composition of the cladding glass of the multicore fiber, the corresponding volume region within the multicore fiber forms part of the optical cladding. In embodiments where the chemical composition of the first glass region of the target rod and the chemical composition of the cladding glass of the multicore fiber are different from each other, additional functions can be added to the target rod within the multicore fiber. For example, the target rod can function as a "stress zone" that generates and / or compensates for radially acting compressive or tensile stresses within the fiber.

[0066] The target rod may be a single unit or may consist of multiple parallel cylinders. In addition to the first glass region, the target rod may have further glass regions or multiple glass regions that differ from the first glass region in terms of chemical composition. In particular, the target rod may include at least one core glass region, the core glass region forming the signal core in the final multicore fiber.

[0067] The target rod is used to provide marker elements made of marker glass in addition to the first glass region. The marker elements exist in the intermediate product, for example, as elongated cavities, or as integrated or non-integrated components made of marker material, or as a coating of such components with marker material, forming continuous linear marker zones or air-filled hollow channels formed of marker material within the multicore fiber.

[0068] The marker element is positioned within or on the target rod. For example, it may be positioned within a hollow channel extending parallel to the longitudinal axis of the cylinder, or attached to the outer surface of the target rod. Although the marker element is positioned within or on the target rod, it is not entirely contained within the cladding layer manufactured by the external deposition method. Therefore, it is not necessary to adapt the cladding layer for the purpose of inserting the marker element, for example, by machining, particularly by forming a separate bore in the cladding layer to receive the marker element. Thus, the risk of damage associated with such adaptation of the cladding layer is eliminated.

[0069] The dimensional stability and straightness of the target rod can be ensured by simple measures. These measures include, for example, machining and / or stretching processes of the target rod. Machining may optionally be performed using external machining methods, which generally reduce complexity significantly compared to internal machining methods.

[0070] The marker element's proximity to the target rod facilitates axial alignment of the marker element. The straightness of the target rod is relatively easy to achieve, which also facilitates axial alignment of the marker element. This is particularly true in a particularly preferred embodiment in which the marker element is positioned between the first glass region and the glass-clad region of the target rod.

[0071] Therefore, the intermediate product is equipped with a marker element, eliminating the need to form a separate bore region in the cladding material layer associated with the risks and difficulties described above. At the same time, high precision can be ensured despite the high aspect ratio, as evidenced by the fact that the deviation of the axial parallelism of the marker element in the intermediate product is less than 0.3 mm / m.

[0072] The target rod may consist of a single glass cylinder or of multiple glass cylinders connected to one another. The multiple glass cylinders may have the same composition or different compositions. The target rod may consist entirely or partially of clad glass, core glass, or another type of glass. For example, the target rod may have a central core made of core glass, which is surrounded by a clad region made of clad glass.

[0073] The marker element exists, for example, as a rod extending parallel to the target rod.

[0074] In further embodiments, the marker element exists as a layer of marker material attached within a hollow channel of the target rod or within a region of the outer surface of the target rod. Here again, the marker element layer and the target rod may optionally exist in an integrated, i.e., fused, form.

[0075] The marker element extends along the longitudinal axis of the target rod, preferably along its entire length, and can be attached to the target rod locally, but preferably continuously.

[0076] By being attached to the target rod, the marker element benefits from its straightness and alignment, and these properties are transferred to the marker element to some extent. This attachment is based, for example, on frictional engagement, integral coupling, and / or shape compatibility between the target rod and the marker element.

[0077] The marker element is preferably designed in the form of a cylindrical component made of marker material, or in the form of a layer connected to a target rod, or a mass made of marker material. At least one cylindrical marker element component is, for example, a tube, preferably a rod. In the case of a marker element in the form of a tube, the tube wall may contain a material with a higher viscosity than clad glass, so that the bore does not completely collapse during the fiber drawing process and is maintained as a cavity ("airline") within the finished multicore fiber.

[0078] The cross-sectional shape of the target rod is generally circular. However, it may also have shapes that deviate from a circle, such as oval, elliptical, or polygonal. The surrounding circle enclosing the cross-sectional contour has a diameter in the range of, for example, 36 mm to 76 mm.

[0079] In a preferred embodiment, the target rod has a recess extending along the longitudinal axis of the target rod, the recess forming a marker element, or a marker element being positioned within the recess.

[0080] The recess is preferably designed as a bore penetrating the target rod, or as a longitudinal groove (longitudinal channel) on the outer surface of the target rod. This forms an air-filled hollow channel, or is filled, for example, with a cylindrical component made of marker material or particulate marker material. The particulate marker material can have a certain degree of dimensional stability by partial compression or the addition of a binder. The marker material fills the recess as completely as possible, thereby ensuring the shape fit between the marker element and the target rod. The marker element and the target rod can also be connected to each other in advance (i.e., before the fabrication of the cladding material layer), for example, by sintering or fusing them together.

[0081] In this regard, embodiments in which longitudinal grooves are provided on the target rod are particularly preferred. This is because, on the one hand, longitudinal grooves are particularly easy to manufacture on the outer surface of the target rod compared to the bore, for example by milling with a mechanical milling machine or by laser ablation. On the other hand, the longitudinal grooves thus manufactured are just as accurate and straight as the target rod itself. Furthermore, the depth or opening width of the longitudinal grooves can be substantially as small as desired, for example, both less than 15 mm, preferably less than 10 mm, and particularly preferably less than 5 mm. This makes it possible to manufacture geometrically accurate marker elements in a small volume in a simple manner, and the marker elements have an axial deviation of less than 0.3 mm / m in the intermediate product, and therefore a small, high-precision marker zone is formed within the multicore fiber accordingly.

[0082] The longitudinal grooves are filled with marker material, for example, by inserting cylindrical components (rods or tubes) made of the marker material, by introducing a bed of marker material particles, or by coating the inside of the longitudinal grooves with the marker material. The cylindrical components, internal coating, or bed formed of the marker material can further be fixed to the longitudinal grooves by fusion.

[0083] The marker element comprises an elongated hollow space (channel) filled with air or a channel, or preferably a marker material having at least one physical and / or chemical property different from the clad glass obtained by vitrifying the clad material layer of the adjacent first glass region of the target rod and / or the intermediate product, the property being selected from refractive index, color, fluorescence, and / or glass specific density.

[0084] Based on the method according to the present invention, or using an intermediate according to the present invention, a multicore fiber is obtained having multiple signal cores and traversed by at least one continuous linear marker zone. The marker zone helps to break symmetry and to clearly identify the signal cores and their positions relative to each other and their positions relative to the fiber's central axis.

[0085] Definition and measurement method The individual terms used in the above description are further defined below. These definitions are part of the description of the present invention. For terms and measurement methods not specifically defined herein, the interpretations of the International Telecommunication Union (ITU) apply. In the event of any inconsistency between one of the following definitions and the remainder of this specification, the statement made elsewhere in this specification shall prevail.

[0086] Clad glass / glass clad region The glass-clad region includes clad glass. At least a portion of the glass-clad region is manufactured by external deposition. The core glass region designed for signal transmission is formed within the glass-clad region. The clad glass consists of, for example, undoped quartz glass or contains at least one dopant that reduces the refractive index of the quartz glass. Fluorine and boron are dopants that can reduce the refractive index of the quartz glass.

[0087] Core rod / core glass area A core rod contains core glass having a radially uniform or non-uniform refractive index profile. The core glass of each core rod forms a core glass region. A core rod may include a region made of core glass having a relatively high refractive index and at least one further region made of another glass having a relatively low refractive index, such as quartz glass doped with fluorine and / or chlorine. The glass with the highest refractive index is generally located on the central axis of the core rod. It consists, for example, of quartz glass to which at least one dopant has been added to increase the refractive index. In a multicore fiber, the core rod forms at least one signal core, and the signal to be transmitted is mainly transmitted within that at least one signal core. The signal core may be adjacent to other glass regions having a lower refractive index, also provided by the core rod.

[0088] Target rod The target rod has at least one first glass region and at least one marker region adjacent to the first glass region and formed within or on the target rod. In addition to the first glass region, the target rod may have further or a plurality of glass regions that differ from the first glass region in terms of chemical composition. The target rod is made of glass that is part of a multicore fiber. In particular, the target rod may have a glass region that functions as a signal core in the final multicore fiber. To impart additional properties to the multicore fiber, the composition of the first glass region may correspond to or deviate from the composition of the clad glass.

[0089] The target rod functions as a deposition mandrel for carrying out an external deposition method to produce the cladding material layer from the target rod. The cladding material layer consists of quartz glass or exists entirely or partially as an SiO2 soot layer. Thus, the target rod helps to insert marker elements into the region of the cladding material layer near the axis, in addition to the target rod material.

[0090] Marker elements / Marker materials / Marker glass The marker element comprises air and / or a marker material, particularly at least one marker glass. The chemical composition of the marker material is different from that of the adjacent first target rod glass region, and / or the density of the marker material is different from that of the clad glass and the first target rod glass region. The marker element is present in the semi-finished product as a component or as a layer or mass on a component, forming an optically detectable marker zone within the multicore fiber.

[0091] Component group / Integrated preform / Secondary preform / Semi-finished product / Intermediate product A “component group” includes a target rod having at least one marker element and a cladding material layer deposited on the target rod having a core rod bore, in each case one core rod is inserted into the core rod bore. By fixing the core rod to the core rod bore, for example by narrowing or collapsing the hollow glass cladding cylinder end, a “preform,” also referred to herein as an “integrated preform,” is obtained. The component group or the (integrated) preform is stretched to form a “secondary preform,” or a multicore fiber is formed directly. Here, the term “semi-finished product” encompasses the component group, the integrated preform, and the secondary preform. The cylindrical bonded composite is called an intermediate product, and it consists of a glass target rod, a marker element, and an SiO2 soot-containing cladding material layer surrounding the target rod and the marker element.

[0092] Quartz glass Here, the quartz glass is a high-siliceous glass having an SiO2 content of at least 80 mol%, preferably at least 90 mol%. The quartz glass is either undoped or contains one or more dopants. The quartz glass is, for example, a melt product from naturally occurring SiO2 raw materials (natural quartz glass), or is manufactured synthetically (synthetic quartz glass), or consists of mixtures of these types of quartz glass. Synthetic clear quartz glass can be obtained, for example, by flame hydrolysis or oxidation of synthetically produced silicon compounds, by polycondensation of organosilicon compounds by the so-called sol-gel method, or by hydrolysis and precipitation of inorganic silicon compounds in liquid.

[0093] Integration / Fusion / Vitrification / Collapse When referring to glass components or SiO2 lumps, fusion is understood to mean that the components or SiO2 lumps melt together on a contact surface. Fusion is achieved by heating the components or SiO2 lumps, at least in the area of ​​the contact surface, using a heat source such as a furnace, burner, or laser. During collapse, the gaps between the components close. Vitrification refers to the hot process of transforming porous soot material into dense glass. Integration can include the fusion, vitrification, and collapse processes. The result is a thermally compressed, easy-to-handle semi-finished product, such as a preform or component group tightly joined by fusion.

[0094] Position display:Top / Bottom These indications relate to the position during the stretching process and / or the fiber drawing process. "Bottom" indicates the position in the direction of the drawing process, and "Top" indicates the position opposite to the direction of the drawing process.

[0095] cross section The cross-section was taken perpendicular to the longitudinal direction / longitudinal axis.

[0096] Longitudinal section The intercepts were taken parallel to the longitudinal axis.

[0097] Boa The terms “bore,” “central bore,” “internal bore,” or “longitudinal bore” refer to a hole having a cylindrical or any other arbitrary internal geometry. These are manufactured, for example, by a drilling process, or by depositing a layer of material onto the outer surface of a mandrel by a deposition or pressing process, and then removing the mandrel.

[0098] Axis parallel alignment / Axis parallelism In all cases, the reference axis is the longitudinal axis of the semi-finished product or preform, or the central axis of the multicore fiber. [Brief explanation of the drawing]

[0099] Exemplary Embodiments The present invention will be described in more detail below with reference to exemplary embodiments and drawings. In particular, in the schematic diagram, [Figure 1] The image shows a cross-sectional view of a semi-finished product for manufacturing a hollow glass-clad cylinder, a core rod, and a multicore fiber having marker elements, based on prior art. [Figure 2] The processing steps (a) to (d) in a modified version of the first method for manufacturing a target rod having a marker element for use as a deposition mandrel in an external deposition method are shown. [Figure 3] The processing steps (a) to (b) in a modified version of the second method for manufacturing a target rod having a marker element for use as a deposition mandrel in an external deposition method are shown. [Figure 4] Figure 1 shows the deposition of the soot layer on the target rod, including the marker element. [Figure 5] This shows a component composite on a target rod, including a marker element and a vitrified soot layer. [Figure 6] Figure 5 shows a cross-section of the component composite after the longitudinal bore has been formed. [Figure 7] Figure 6 shows the integrated preform of the component composite, in which the core rod is inserted into and fused in the longitudinal bore. [Figure 8]Further embodiments of the integrated preform are shown in cross-section.

[0100] Figure 1 schematically shows a cross-section of a prior art clad glass hollow cylinder 1 that serves as a substrate for the manufacture of multicore fibers. The hollow cylinder 10 is manufactured by a known method using the OVD method. In this method, SiO2 soot particles are deposited from the gas phase onto the outer surface of a cylindrical deposition mandrel that rotates around its longitudinal axis, resulting in the formation of an SiO2 soot body on the outer surface of the deposition mandrel. After the completion of the deposition process, the deposition mandrel is removed, leaving an internal bore 20. The SiO2 soot body is then vitrified to form the hollow cylinder 10. Four longitudinal bores 40 for receiving core rods 30, and a smaller longitudinal bore 60 for receiving a marker rod 70 are introduced into the wall of the hollow cylinder 10. During the integration of this component group, a filling rod 80 is inserted into the central bore 20, which is made of the same material as the hollow cylinder clad glass, for example.

[0101] The multiple longitudinal bores (40, 60) within the hollow cylinder clad glass involve considerable effort and a risk of failure. In particular, when manufacturing the smaller longitudinal bore 60 to accommodate the marker rod 70, unacceptable deviations from axial parallelism are likely to occur, potentially leading to cracks in the bore's inner wall, which can result in the failure of the hollow cylinder 1, which is manufactured in a complex manner. Filling and collapsing the central bore 20, as well as the custom manufacturing of the filling rod 80, also involve effort and a risk of failure, and can easily cause dimensional deviations. These drawbacks are avoided in the method of the present invention, which will be described below with reference to Figures 2 to 8.

[0102] Figures 2 and 3 schematically illustrate the method steps for manufacturing a target rod 1 having a marker element 5 or 5a.

[0103] The target rod 1, schematically shown in Figure 2a, is made of synthetically produced undoped quartz glass, commercially available under the name F300. Known techniques such as VAD (Vapor-Axis Deposition), OVD (External Vapor Deposition), MCVD (Modified Chemical Vapor Deposition), or powder pressing are suitable for this purpose. It functions as a deposition mandrel in the OVD external deposition method, which will be further described below with reference to Figure 4. The target rod 1 is approximately 1800 mm long and has an outer diameter of approximately 42 mm, achieved by circular grinding. Circular grinding eliminates any possible irregularities and bends on the outer surface 2. Alternatively or additionally, diameter adjustment and surface improvement are achieved by stretching in a toolless stretching process.

[0104] Figure 2b shows that the longitudinal groove 3 has been milled from the outer surface of the target rod 1. The longitudinal groove 3 extends along the entire length of the target rod 1. It has a U-shape and has a rounded bottom and straight side walls. Its opening width and depth are 6 mm in all cases. In subsequent method steps, a hollow channel can be fabricated from the longitudinal groove 3 to form a marker element within the scope of the present invention.

[0105] Figure 2c shows the marker rod 4 inserted into the longitudinal groove 3. The diameter of the marker rod 4 is 5 mm. It is made of fluorine-doped synthetically produced quartz glass, which is commercially available under the designation F320. Both the viscosity and refractive index of the fluorine-doped quartz glass of the marker rod 4 are lower than those of the undoped quartz glass that constitutes the target rod 1. The marker rod 4 is obtained by stretching a starting cylinder made of F320 quartz glass using a toolless method. It has a smooth surface formed within the molten mass, is characterized by high dimensional stability and straightness, and can therefore be accurately fitted and inserted into the narrow longitudinal groove 3 without difficulty.

[0106] The marker rod 4 inserted into the longitudinal groove 3 is heated along its entire length with a burner, causing the fluorine-doped quartz glass to soften and deform due to its relatively low viscosity. Figure 2d shows the marker glass mass 5 after softening, deformation, and fusion with the target rod 1. The glass volume of the original marker rod 4 matches the internal volume of the longitudinal groove 3, and as a result, the marker glass mass 5 completely fills the longitudinal groove 3.

[0107] In the alternative procedure shown in Figure 3, the manufacturing of the longitudinal groove is omitted on the one hand, and on the other hand, the composite of the target rod 1 and the marker element 5a is manufactured based on the upstream stretching process. As shown in Figure 3a, the starting or pre-product for the stretching process is, on the one hand, a target rod starting cylinder 1a made of undoped quartz glass (F300) having a diameter of 63 mm and a length of 800 mm, and on the other hand, a non-circular starting marker rod 4a whose outer peripheral edge is flat or slightly concave (curved inward) in cross-section. The starting marker rod 4a is made of fluorine-doped quartz glass (F320) and has a diameter of 8 mm. The starting marker rod 4a is fused to the starting target rod 1a in parallel axis alignment, and the aforementioned peripheral edge rests on the outer surface 2a of the target rod. During stretching, the composite of the starting target rod 1a and the marker rod 4a is drawn to a length of 1800 mm. In this case, the starting cylinders (1a, 4a) further fuse with each other, and the low-viscosity quartz glass of the marker rod 4a flows slightly onto the side surface 2 of the target rod, forming a flat glass bead 5a that, after cooling, is fixed and connected to the outer surface 2 of the target rod 1. This is schematically shown in Figure 3b. The stretching process reduces, on the one hand, any geometric errors and dimensional deviations of the starting cylinders (1a, 4a), and on the other hand, directs the final molten composite (1, 5a) straight forward.

[0108] Figure 4 schematically illustrates the use of the target rod 1, thus prepared and filled with marker glass blocks 5, as a deposition mandrel in the OVD (Outer Vapor Deposition) method. Here, a high-purity SiO2 starting material, such as silicon tetrachloride, is supplied to a deposition burner 9 and burner flame 8 and converted into solid SiO2 particles 11. These SiO2 particles 11 are deposited from the gas phase onto the outer surface 2 of the target rod 1, which rotates around its longitudinal axis 1b (direction arrow R), and the deposition burner 9 performs a reverse forward and backward movement along the longitudinal axis 1b of the target rod (which extends perpendicular to the sheet plane in the cross-sectional view of Figure 4). An SiO2 soot layer 17 is formed on the outer surface 2 of the target rod 1. The intermediate product 18 obtained after the completion of the OVD method includes the target rod 1, the marker glass blocks 5, and a cylindrical bonded composite formed from the SiO2 soot layer 17 surrounding the target rod 1 and the marker glass blocks 5.

[0109] The intermediate product 18 is dehydrated in a chlorine-containing atmosphere at a temperature of 850°C, and immediately thereafter, the SiO2 soot layer 17 is vitrified under vacuum at a temperature of 1450°C.

[0110] Figure 5 shows the composite 15 subsequently obtained and formed from the target rod 1, which includes a marker glass block 5 and a glass-clad region 12 formed from undoped synthetic quartz glass and obtained after integration of the SiO2 soot layer. The dashed circular line 12c represents the circumference of the target rod 1. The outer diameter of the composite 15 is set to a value of 200 mm by external round grinding. The glass-clad region 12 extends along the outer surface 2 of the target rod 1, and its usable length, excluding the round end cap, is approximately 1500 mm.

[0111] Figure 6 shows the composite 15 formed from the glass cladding layer 12 and the prepared target rod 1 after four bores 13 are formed in a predetermined (here, square) configuration by mechanical drilling in the direction of the longitudinal axis 1b of the target rod within the glass cladding region 12. The bores 13 serve to receive the core rod 14 (Figure 7) and have a diameter of 30 mm. The bores 13 extend over the entire usable length of the glass cladding region 12 (through bores). In an alternative embodiment, the bores are designed as no-bores.

[0112] Furthermore, four core rods 14 made of Ge-doped quartz glass, each approximately 1500 mm in length and 28 mm in outer diameter, are manufactured. Known techniques, such as MCVD (Advanced Chemical Vapor Deposition), are also suitable for this purpose.

[0113] The core rod 14 is inserted into the bore 13. Next, the component group formed from the composite 15 and the core rod 14 is heated, so that the annular gap around the core rod 14 closes and all the components of the group fuse together. Figure 7 schematically shows the thus integrated preform 16, which consists of the composite 15 of the target rod 1, the marker glass mass 5, the glass clad region 12, and also the core rod 14.

[0114] Next, the integrated preform 16 is stretched to form a secondary preform. In this case, the preform 16 is held in the stretching device by a holder with the longitudinal axis 1b of the target rod vertically aligned. The secondary preform thus produced is finally drawn in a wire drawing device using a conventional method to form a multicore fiber.

[0115] In this exemplary embodiment, the marker element exists as a marker glass block 5 produced by reshaping the original marker rod 4 within the longitudinal groove 3. In an alternative procedure, the capillary is inserted into the longitudinal groove 3 during the external deposition method. During the subsequent integration process, the complete collapse of the capillary is prevented by generating and maintaining overpressure within the capillary. In this way, a cavity is formed that exists within the multicore fiber as an air-filled hollow channel extending along the longitudinal axis 1b. Because the refractive index of air is significantly different from that of the clad glass, the hollow channel can function as a marker zone.

[0116] Except for a smaller radius, the cross-section of the multicore fiber substantially corresponds to the cross-section of the integrated preform 16. The core glass region (14) of the original core rod forms a signal core extending along the longitudinal axis of the fiber, the original target rod (1) forms part of the glass cladding region, and the original marker element (5) forms a visually easily detectable marker zone. The marker zone (5) is characterized by its small size, and therefore the stress on the multicore fiber during the fiber drawing process is small, resulting in less fiber curl.

[0117] In contrast to Figure 7, in the embodiment of the integrated preform 26 for multicore fiber shown in Figure 8, the center of the target rod 1 is occupied by the core rod 14a. The bore for receiving the core rod 14a is formed in one operation along with the other core rod bore 13 (Figure 6) after the soot layer has been vitrified, within and along the longitudinal axis 1b of the target rod. All core rods 14, 14a have the same diameter. Thus, the original target rod 1 has a core glass region, which is surrounded by a glass cladding region adjacent to the marker zone 5.

Claims

1. A method for manufacturing a multicore fiber having a marker zone or a preform for such a multicore fiber, comprising forming a semi-finished product comprising: a glass-clad region made of clad glass in which a plurality of core glass regions made of core glass are embedded; and at least one marker element, wherein the multicore fiber or the preform is obtained by stretching the semi-finished product, and the manufacturing of the glass-clad region comprises a method step of depositing a clad material layer on the outer surface of a target rod having the longitudinal axis of the target rod using an external deposition method, wherein the target rod comprises a first glass region extending along the longitudinal axis of the target rod; and a marker region extending along the longitudinal axis of the target rod and adjacent to the first glass region, wherein the marker region includes the marker element, or provides a hollow channel designed to form the marker element, or to receive the marker element, and wherein the semi-finished product comprises the clad material layer and the target rod.

2. The method according to claim 1, characterized in that the marker region is arranged between the first glass region and the glass clad region.

3. The method according to claim 1 or 2, characterized in that the marker element forms at least one cylindrical component, layer, or mass connected to the target rod.

4. The method according to claim 3, characterized in that the target rod includes a recess extending along the longitudinal axis of the target rod, the recess forms the marker element, or the marker element is positioned within the recess.

5. The method according to claim 4, characterized in that the recess includes a longitudinal groove on the outer surface of the target rod.

6. The method according to any one of claims 3 to 5, characterized in that the marker element is attached to the target rod before carrying out the external deposition method.

7. The cladding material layer produced using the external deposition method is SiO ２ The method according to any one or more of claims 1 to 6, characterized in that it is formed as a soot layer based on the above.

8. The method according to claim 7, characterized in that the soot layer is vitrified by heating to a vitrification temperature.

9. The method according to claim 8, characterized in that, prior to vitrification, the soot layer undergoes a doping treatment in an atmosphere containing at least one dopant, and / or a dehydration treatment in a halogen-containing atmosphere or under vacuum.

10. The method according to claim 7 or 8, characterized in that a core rod bore for receiving a core glass rod is manufactured after the soot layer has been vitrified.

11. The method according to any one or more of claims 1 to 10, characterized in that the marker element is designed as a hollow channel, or comprises a marker material that differs from the adjacent clad glass and / or the adjacent first glass region of the target rod in at least one physical and / or chemical property selected from refractive index, color, fluorescence, and / or glass specific density.

12. The manufacturing of the aforementioned semi-finished product is carried out in the following method steps: (a) Providing the target rod including the marker region, (b) The step of providing a plurality of core rods including core glass, (c) The step of depositing the cladding material layer on the outer surface of the target rod using the external deposition method described above, (d) The step of forming a core rod bore in at least the cladding material layer and optionally in the first glass region of the target rod, (e) The method according to any one or more of claims 1 to 11, comprising the step of inserting the core rod into the core rod bore.

13. The method according to claim 12, characterized in that the marker region is designed as a hollow channel, and a cylindrical marker element is inserted into the hollow channel before or after insertion of the core rod.

14. An intermediate product for manufacturing a multicore fiber or its preform, comprising a glass target rod, a marker element formed on or within the target rod, and SiO ２ An intermediate product comprising a clad material layer containing a soot, wherein the layer surrounds the target rod and the marker element and is connected to the target rod in a friction engagement, shape-fitting and / or integral coupling manner.

15. The intermediate product according to claim 14, wherein the target rod includes a first glass region adjacent to the marker element, the marker element is preferably positioned between the first glass region and the cladding material layer, and very preferably at least partially fills a recess on the outer surface of the target rod.

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